C++ Lesson: Friend Functions

Introduction

Friend functions allow access to private and protected members of a class while not being a member function.

Syntax


#include <iostream>
using namespace std;

class MyClass {
private:
    int data;
public:
    MyClass(int val) : data(val) {}
    friend void displayData(MyClass obj);
};

void displayData(MyClass obj) {
    cout << "Private Data: " << obj.data << endl;
}

int main() {
    MyClass obj(42);
    displayData(obj);
    return 0;
}

Friend Function with Multiple Classes


#include <iostream>
using namespace std;

class ClassB;
class ClassA {
private:
    int numA;
public:
    ClassA(int val) : numA(val) {}
    friend void add(ClassA, ClassB);
};

class ClassB {
private:
    int numB;
public:
    ClassB(int val) : numB(val) {}
    friend void add(ClassA, ClassB);
};

void add(ClassA objA, ClassB objB) {
    cout << "Sum: " << objA.numA + objB.numB << endl;
}

int main() {
    ClassA a(10);
    ClassB b(20);
    add(a, b);
    return 0;
}

Advantages & Disadvantages

Advantages: Increased flexibility, operator overloading, access to multiple classes.
Disadvantages: Breaks encapsulation, not inherited, maintenance issues.